1. Field of the Invention
The present invention relates to micro-miniature gas chromatographs and more particularly to spiral solid phase columns in silicon wafers and their fabrication.
2. Description of Related Art
Gas chromatographs are used by various scientific laboratories and government law enforcement agencies to analyze the chemical makeup of samples of materials. Some such instruments are able to reliably analyze samples where the constituents are concentrated as low as one part per million. Prior art equipment can provide useful results when as little as a few microliters of gas has been "sniffed". But such equipment is extraordinarily bulky and too delicate to be called portable.
Gas chromatographs generally comprise three basic parts, an injector, a column, and a detector. The column generally comprises a tube filled with a solid phase, through with a carrier phase must migrate. Gas samples are carried into a column by a carrier gas such as hydrogen or helium. The propagation front of the gas and how it diffuses into and back out of the column itself are highly dependent on the inside cross-sectional geometry of the column and the characteristics of the individual molecules in the gas. A circular cross-section column is ideal and produces the best results, as the lighter molecules of sample gas will diffuse faster than the heavier molecules and a circular column presents a uniform path, regardless of the direction of outward diffusion into the column walls.
The prior art has not succeeded in the fabrication of circular cross-section microcapillaries in silicon for use in GC columns. Conventional attempts to fabricate columns have resulted in squared-bottom trenches in one wafer that are capped by another wafer, typically some second, different material that produces thermal coefficient-of expansion problems.
Silicon wafers are flat and are universally processed from one side, e.g., with depositions, implants, masks and etching. Semiconductor fabrication processes are conventionally used to create non-electronic microstructures using silicon, oxides and metals.
Because of the way masks must be used with etchants, it is practically impossible to create a round tube or capillary in a single wafer of silicon. Two obstacles are encountered. First, the etching of half-round microchannels in silicon wafers has proven impossible with conventional methods. And second, bonding together matching wafers in a sandwich with the completed round capillary in between has proven to be very tedious. The conventional art in matching the geometries of features on each of the two respective halves is very precise, but the bonding together of two separate silicon wafers does not lend itself to such easy precision. Mis-registrations of even a few mils can cause ruinous overlaps of the two halves of ten to twenty percent and more.